The access to double-data-rate (or DDR) memories, such as for example synchronous dynamic RAM memories (SDRAM) with double-data-rate, is generally implemented by a parallel high-data-rate link.
The writing and the reading of data in a DDR memory are usually carried out by the transfer of a group of several words to or from consecutive memory locations, each word being transferred in one cycle of a clock signal.
Each word comprises several bits, generally 16 or 32 bits, and is transferred in one clock cycle over a parallel link comprising as many bit-lines. The transfer (or “burst” according to the term commonly employed) comprises for example 8 consecutive words in BL8 (for ‘Burst Length 8’) protocol or 16 consecutive words in BL16.
Thus, over any given bit-line of the link, a horizontal pattern may be repeated from one transfer to another, notably in the case of a repeated storing of the same data.
Such a horizontal repetition, in other words a repetition of the same pattern over a bit-line of the link, may generate, by resonance, abrupt increases in the spectral density of power radiated by the high-data-rate parallel link.
Owing to the high data transfer frequencies implemented in DDR memories, these energy spikes can cause electromagnetic interference (or EMI) effects at frequencies used by neighboring electronic systems, such as usually a radiofrequency telecommunications antenna, for example of the Wi-Fi type, or any other system.
The current solutions for attenuating EMI effects have the drawbacks of reducing the transmission performance characteristics, such as for example the ‘slew rate’ control, of only being effective for the clock signals, such as for example ‘spread spectrum’ techniques, or else of being costly to produce, such as for example electromagnetic shielding.
A solution for scrambling transferred data, aimed at reducing the variations in the power supply by vertically scrambling the bits of each word according to a balanced distribution of “0”s and of “1”s has been provided in the Patent application US 2009/0086972 A1, but does not address the problem of electromagnetic interference effects generated by a horizontal repetition of a pattern of data.